The present invention relates generally to an oxygen sensor for detecting the oxygen concentration of a measurement gas, especially an exhaust gas produced by an internal combustion engine. More particularly, the invention is concerned with such an oxygen sensor which has an elongate sensing element having an oxygen detecting portion at its one end, and which is simple in construction and easy to manufacture and assemble.
There has been known an oxygen sensor which detects or determines the oxygen concentration of an exhaust gas emitted from internal combustion engines, for the purpose of controlling the combustion or fuel burning conditions of the engine according to signals produced by the oxygen sensor, and thereby purifying the exhaust gas and saving the fuel consumption of the engine. An example of such oxygen sensors uses a sensing element which comprises a body of oxygen-ion conductive solid electrolyte such as zirconium oxide doped with calcium oxide or yttrium oxide, and further comprises suitable electrodes disposed on opposite surfaces of the solid electrolyte body. In this oxygen sensor, one of the electrodes is exposed to a reference gas while the other electrode is exposed to the exhaust gas. In operation, the oxygen sensor produces an output signal which represents an electromotive force induced between the two electrodes according to the principle of an oxygen concentration cell. In recent years, there has been an increasing tendency to use an elongate planar sensing element rather than a conventionally used tubular sensing element, in view of ease of manufacture and structural simplicity of the sensor. Such an elongate planar sensing element has, at its one end, an oxygen detecting portion to be exposed to an exhaust gas or other measurement gas.
Oxygen sensors of various types as introduced above are installed such that their sensing element extends through the wall of a conduit through which an exhaust gas or other measurement gas is caused to flow, so that the end portion of the sensing element is inserted into the conduit and its detecting portion is exposed to the measurement gas in the measurement-space in the conduit. Generally, the oxygen sensor employs a first protective covering member, usually of cylindrical shape in cross section, which encloses the periphery of the end portion of the sensing element inserted in the fluid conduit, in order to protect the sensing element against thermal shock by high-temperature exhaust gas, to prevent particles in the exhaust gas from being deposited on the sensing element, and for other protective purposes. The oxygen sensor further employs a second protective covering member also of cylindrical shape which accommodates the exposed portion of the sensing element outside of the fluid conduit, in order to protect that exposed portion from outside foreign substances including liquids such as water.
In such a known oxygen sensor wherein the first and second protective covering members are separate parts, the structure for supporting various components of the sensor within the separate covering members is necessarily complicated, and accordingly the procedure to assemble the components into a sensor unit is cumbersome. In other words, an oxygen sensor using such separate protective covering member is relatively difficult to manufacture. In addition, the oxygen sensor is required to have complicated structures for attachment to the fluid conduit in such a manner as to ensure fluid-tight sealing at the first and second protective covering members, for preventing an exhaust gas or other measurement gas from leaking outside, and for protecting the sensor against entry of water or other liquids which would damage the sensing element as it operates at a high temperature. This sealing structure will lead to increased difficulty in the manufacture and assembling of the oxygen sensor.